That Son Agro is the epitome of a start to finish HID light from what I've read and been told. It's a 'one bulb' version of best of a MH and HPS without having to switch between two bulbs or have two ballasts. I put mine under an EYE Hortilux bulb (HPS400w) from the point I got it set up on. Which was about two weeks in to growing. Again, the only reason I'm using that Eye Hortilux and not what you have, is because my hydro shop can't manage to keep the Son Agro's in stock and were out that day and I wanted to get going ASAP being that my plants had already sprouted. And the EYE Hortilux was the closest thing to the Son Agro they had in stock.
I'd keep your little ones going under the HPS and hell, the more light the better from I can tell. Use those T5's as side lighting.
With the weather cooling down you'll most likely be alright when it comes to heat. Put them on an 18/6 to 20/4 lighting schedule and have the 'off time' happen during the hottest part of the day.. Oh, and fans, fans will help quite a bit when it comes to that. I've got a $10 end of summer special from Home Depot. My room has no air conditioner or heater and gets no hotter than upper 70's and no cooler than lower 70's. You're not going to have the heat issues you would with a 1000w bulb by any means. The big problem I ran in to was how far up from the plants to put that light... I've seen everything from one to four feet. You can ask that question here and every answer will be different. My light housing has tempered glass under the bulb and I keep it between 16 and 18 inches off the top of the plants. I came to that height by matching the heat given off on the back of my hand just above the plant tops to what it would be on a sunny day. That very well might not be the best way and it might not work for you... I don't have enough experience to advise you on that. But, others do!
As for the power consumption issue. Depending on what your electric company charges you per kilowatt hour, you're looking at an extra 15 to 30 bucks a month on your electric bill. I'm a very paranoid person when it comes to such things, things that are out of my control, like meter readers telling the cops about how much juice I'm using... So, for a few month prior to doing this, I used a little bit more electricity than normal (kept the A/C a bit cooler, left a porch light on at night, didn't bitch when the little lady held the refrigerator door open too long) and when I was ready to roll with my new light, I replaced every normal light bulb in the house with CFL's which took my total 'normal' light usage down from just over a possible 2500w to about 600w and stopped leaving my computer (which has a 450w power supply) on 24/7. 400 Watts isn't all that hard to hide and it's not that expensive. Again, I'm not an expert. But, from what I've read about the 'electric bill can get you busted' thing is that... A) 9 times out of 10 they already have to be looking at you before they check that and that 10th one is some jack ass at the power company trying to help a friend or relative out that's a cop. And B) It's a large spike that will get you noticed. Look at it like this, you ever have one of those six-foot tall torch lamps that were big in the 80's & 90's, those things were 500w at full brightness. For folks like you and me (small timers) a little cutting back here and there will more than hide a 400w bulb. As for the T5's, you can offset that by making your morning toast in the broiler of a gas stove in stead of the electric toaster.
Not knowing any of the finer points of your set up and me sill being a VERY rank amateur, the only other problem I can for see that you'll have to deal with is SMELL! Pour over this site and you'll find a wealth of information on carbon scrubbers and how to make them for a few dollars.
Good luck and be safe!
